The hollow fiber membranes have an advantage in that a substance exchange such as a gas exchange through a membrane is achieved in a high efficiency, and are therefore utilized in various fields for exchanging substances.
By way of example, an artificial lung utilizing the hollow fiber membranes is so designed that, with the use of microporous hollow fiber membranes made of hydrophobic polymer such as polyolefin or gas-permeable homogeneous hollow fiber membranes made of, for example, silicone, gas exchange takes place between oxygen-containing gas and blood through membranes, and is available in two types; an intracapillary-flow type in which the gas flows exteriorly of the hollow fiber membranes while the blood flows through hollow part of the hollow fiber membranes and an extracapillary-flow type in which, in a sense opposite to the intracapillary-flow type, the gas flows through hollow part of the hollow fiber membranes while the blood flows exteriorly of the hollow fiber membranes.
In the intracapillary-flow type, although no channeling (biased flow) of the blood flow occur substantially if the blood is supplied so as to be uniformly distributed to the multiplicity of the hollow fiber membranes, the blood flowing through the hollow part of the hollow fiber membranes represents a laminar flow and, therefore, the hollow fiber membranes must have a reduced inner diameter in order to increase the oxygenating ability (oxygen transfer rate per unitary membrane surface area). In view of this, the hollow fiber membranes having an inner diameter of about 150 to 300 .mu.m have been developed for use in an artificial lung. However, even though the diameter is so reduced, the oxygenating ability does not increase remarkably so long as the blood flows in a laminar fashion. Moreover, the smaller the inner diameter of the hollow fiber membranes, the more does the clogging (choking of the inside of the hollow fiber membranes due to blood clotting) occurs frequently, posing an obstruction to the practical use.
On the other hand, generally in the artificial lung, several tons of thousands of hollow fiber membranes are bundled and, because of this, special care is required to supply the gas so as to be distributed sufficiently and uniformly over the individual outer walls of the hollow fiber membranes. In the event that the gas is not sufficiently distributed, the carbon dioxide removing ability (CO.sub.2 transfer rate per unitary membrane surface area) is lowered.
On the other hand, the extracapillary-flow type is superior to the intracapillary-flow type in that the gas can be distributed satisfactorily accompanied by a minimized loss of pressure of the liquid and, also, it can be easily structured to cause a turbulence in the flow of the blood.
In the extracapillary-flow type, the internal structure of the blood passage has close relations to a shortage of oxygenation due to blood channeling and an occurrence of blood clotting due to stagnation of blood flow, and therefore, proper selection of the structure of the blood passage is extremely important in avoiding these problems.
The extracapillary-flow type artificial lung module comprising the hollow fiber membranes and having a structure to prevent an occurrence of the channeling in the blood flow hither to known includes a module in which baffle plates are disposed in the blood passage.
By way of example, as shown in FIGS. 6A and 6B, the Japanese Laid-open Patent Publication No. 60-193469 discloses an extracapillary-flow type artificial lung module comprising the hollow fiber membranes of a structure having a contacting chamber 21 of a generally elongated rectangular cross-section in which there is defined a plurality of cells 20 separated by blood passages 18 that are constricted by baffle plates 17. In these figures, reference characters X, Y and Z represents the height, the width and the length of each cell 20, respectively.
On the other hand, in applying the substance exchanger apparatus comprising the hollow fiber membranes in various fields, a variety of demands have been proposed, and among them, a reduction in size to provide a compact apparatus is eagerly longed for.
By way of example, an artificial lung compact in size, not of a large size for installation in hospitals or similar establishments, is longed for. In other words, when made compact in size, the artificial lung will be convenient to carry and can quickly be provided for in emergency situations such as an occurrence of acute respiratory failure. Further, when the volume of the apparatus is reduced, the amount of liquid primed in the apparatus can advantageously be minimized enough to lessen the harmful effect which a patient may suffer from, for example, as a result of dilution of the patient's blood with the priming liquid.
The artificial lung thus made compact in size is disclosed in the Japanese Laid-open Patent Publication No. 2-41172, which comprises a housing including outer and inner barrels to define coaxial outer and inner storage spaces, a blood treatment unit disposed within the outer storage space of the housing, and a blood supply mechanism disposed within the inner storage space of the housing. This artificial lung employs the hollow fiber membranes in the blood treatment unit, and is constructed so that blood to be treated can flow exclusively along the axial direction of the hollow fiber membranes.
When the substance exchanger apparatus comprising the hollow fiber membranes is to be made compact in size, the volume of the apparatus in which the hollow fiber membranes are disposed is necessarily reduced, accompanied by a reduction in total membrane surface area. Accordingly, in making the apparatus compact in size, it is necessary to raise the efficiency in exchanging substances for the sake of a satisfactory substance exchange with the reduced total membrane surface area. By way of example, in the case of the artificial lung, the structure of the apparatus must be made so that the blood can be supplied to the outer walls of the hollow fiber membranes efficiently enough to prevent an occurrence of channeling and the blood oxygenated after the gas exchange can be quickly removed from the outer walls of the hollow fiber membranes and the surroundings thereof.
In the hollow fiber membrane-type artificial lung module comprising the baffle plates as shown in FIG. 6, the occurrence of the channeling in the blood flow is not necessarily eliminated sufficiently, and it involves a problem in that the function of the artificial lung tends to be lowered as a result of an occurrence of thrombus at an area where stagnation takes place in blood flow. In addition, the structure of the artificial lung shown in FIG. 6 is not necessarily considered suited for making the artificial lung compact in size.
Even in the compact artificial lung disclosed in the above mentioned Japanese Laid-open Patent Publication No. 2-41172, the blood flows axially of the hollow fiber membranes. Therefore, the problem of channeling in the blood flow is not necessarily be eliminated sufficiently and the possibility cannot be denied that, with passage of time, the gas exchange efficiency tends to be lowered and/or thrombus is apt to occur at the area where blood stagnation takes place.
In view of the foregoing, an object of the present invention is to provide a substance exchange apparatus comprising the hollow fiber membrane, which exhibits a high substance exchange efficiency and which is suited for fabrication in a compact size.